This invention relates to a packaged electronic structure, and, more particularly, to such a structure wherein the packaging of an electronic device is made of aluminum nitride/aluminum composite material.
Microelectronic devices are used in a wide variety of applications. Such devices typically include a generally planar piece (commonly termed a xe2x80x9cchipxe2x80x9d) of a material such as silicon, upon which circuit elements are mounted. The circuit elements can include both passive elements such as resistors or capacitors, and active elements such as transistors or sensors. Many types of microelectronic devices can be prepared using existing fabrication technology. The microelectronic device has the advantages of small size and light weight.
The chip and its circuit elements, considered alone, have some disadvantages. They are rather fragile and sensitive to environmental degradation. Attachment of external leads can be difficult due to the small size. The chip and its circuit elements can also generate a great deal of heat in a small area, leading to overheating of the device. Most of the available types of circuit elements are particularly sensitive to overheating.
In an effort to overcome these disadvantages, support and packaging structures have been developed. The chip may be attached to a stronger substrate made of a material such as aluminum oxide, which becomes a part of the electronic device. The electronic device may also be fixed to, and protected by, a package structure. A common type of package structure includes a base to which the substrate or chip of the electronic device is fixed and a physically and thermally compatible lid which attaches over the base and affixed electronic device. Such a package can be hermetically sealed to prevent environmental damage to the electronic device, but can also provide feedthroughs for external leads to reach to the electronic device. In other cases, only a base is used.
The package must be designed so that its presence does not adversely affect the operation of the microelectronic device. Most commonly, this requirement means that the thermal expansion coefficient of the package base must be comparable with that of the portion of the electronic device that is affixed to it. If the thermal expansion coefficients are markedly different, temperature changes during service of the electronic device can lead to thermally induced stresses in the fragile chip and to the damage of the electronic device.
The package used for some applications, particularly those involving high-power electronic devices, should also have at least moderate thermal conductivity. If the package is too good an insulator, the heat produced within the package cannot be dissipated. Excessive temperature increases are the result. Lastly, the packaging desirably should be made of a relatively light weight material, as any excess weight partially negates the size and weight advantages otherwise inherent in microelectronic devices.
At the present time, metals such as molybdenum, tungsten, invar, and kovar are used as the housing or packaging materials. Nonmetals such as aluminum oxide are used as the substrate materials. None of the packaging materials has an optimal combination of properties for use with silicon chip devices. Invar and kovar have acceptable coefficients of thermal expansion, but have high densities and low thermal conductivities. Molybdenum has a high coefficient of thermal expansion and high density, but acceptable thermal conductivity. Tungsten has an acceptable coefficient of thermal expansion and thermal conductivity, but is very dense. Aluminum oxide has an acceptable density and coefficient of thermal expansion, but very low thermal conductivity.
There is a need for an improved packaging material for use with electronic devices. The packaging material should have the combination of desirable properties that make it suitable for use with common types of electronic chip devices. The present invention fulfills this need, and further provides related advantages.
The present invention provides a packaged electronic structure in which the package can be manufactured so that its coefficient of thermal expansion is matched to that of the electronic device. The material of construction of the package combines the features of metals and non-metals into a single composite material, and has the most desirable thermal, mechanical, and electrical properties of such a material for use in the packaging application. The package is readily formed from its material of construction and is stable in long-term service.
In accordance with the invention, a packaged electronic structure comprises an electronic device, and a package to which the electronic device is affixed. At least a portion of the package comprises a composite material made of aluminum nitride dispersed in aluminum.
The package is preferably prepared from aluminum nitride and aluminum powders by sintering, most preferably by hot isostatic pressing. Selected amounts of the powders are mixed together and cold pressed to form a xe2x80x9cgreen compactxe2x80x9d. The green compact is placed into a mold. A hot isostatic pressing medium is placed around the green compact. The assembly is heated to elevated temperature, preferably just above the melting point of the aluminum powder, and isostatically compacted to a final, dense or nearly fully dense, structure. The composite material may be compacted to the required final shape, or it may be compacted to a semi-finished shape and then machined or formed to the final shape.
Aluminum and aluminum nitride both have relatively low densities, so that the weight of the final package is low. The coefficient of thermal expansion of the package can be selected by adjusting the relative proportions of the aluminum nitride powder and the aluminum powder. The more aluminum nitride powder that is used in the mixture and thence is present in the final package, the lower the coefficient of thermal expansion. The coefficient of thermal expansion of the package can thereby be selected to match that of the microelectronic device chip or substrate, or have a slightly different value if desired. The thermal conductivity of the composite material is high, as both aluminum and aluminum nitride have high thermal conductivities. The weight and thermal conductivity of the composite also vary somewhat with the proportions of the powders used, but that variation is relatively small and is acceptable for all proportions of interest.
The packaged electronic structure of the invention provides an important advance in the art. The weight, thermal conductivity, stability, and fabricability of the package are all excellent. Additionally, the thermal expansion coefficient of the package can be tailored to provide a desired relationship to that of the packaged electronic device. Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention.